As is known, drive units are currently being tested in which the actuation of the intake and exhaust valves is managed by using actuators of electromagnetic type that replace purely mechanical distribution systems (camshafts). While conventional distribution systems make it necessary to define a valve lift profile that represents an acceptable compromise between all the possible operating conditions of the engine, the use of an electromagnetically controlled distribution system makes it possible to vary the phasing as a function of the engine point in order to obtain an optimum performance in any operating condition.
The increase in efficiency and the actual savings resulting from the use of actuators of electromagnetic type are closely linked to the systems and methods used for the control of these actuators.
According to known methods, based for instance on open loop control systems, when each valve is opened or closed, the corresponding actuators are supplied with currents and/or voltages of a value such as to ensure that the valve, irrespective of the resistance opposing it, reaches the desired position within a predetermined time interval.
These methods have some drawbacks.
In the first place, the valves are subject to impacts each time that they come into contact with fixed members in the position of maximum opening (lower contact) or in the closed position (upper contact). This is particularly critical, since the valves are subject to an extremely high number of opening and closing cycles and therefore wear very rapidly.
The fact that the electrical power supplied must always be sufficient to overcome the maximum resistance that the valve may encounter, even though the operating conditions are such that the actual resistance opposing the valve is lower, is also a drawback. In this way, the overall efficiency of the drive unit is reduced as a result of the waste of electrical power.
It is also particularly important that a robust control is implemented so as to enable the intake and exhaust valves to be actuated according to desired movement and timing profiles, irrespective of the disturbances that take place and cause the actual operating conditions to deviate from the nominal conditions. The occurrence of a wide range of phenomena may make the actual operating conditions extremely variable.
For instance, engine temperature variations cause expansions and contractions of materials, as a result of which the friction encountered by the valves may change. Moreover, since the force applied to the ferromagnetic members on which the electromagnets act depends in a highly non-linear manner on the distance between these ferromagnetic members and the polar heads, it will be appreciated that the volume variations caused by heat gradients may have an adverse effect on the control. Further disturbances are due to the fact that the resistance encountered by the valves also depends on the pressure in the combustion chamber which varies depending, for instance, on the torque and power requirement of the consumer and on the engine control strategies implemented.